In motor-driven vehicles as exemplified by an EV (Electric Vehicle) and an HEV (Hybrid Electric Vehicle), power conversion devices are used each including, for example, a high voltage battery, a bidirectional converter, a smoothing capacitor, and an inverter. At the time of powering operation, the power of the high-voltage battery is stepped up by the bidirectional converter, so that DC power is supplied to the inverter, and then AC power converted in the inverter is supplied to a motor. Meanwhile, at the time of regenerative operation, the motor is operated as a power generator, so that DC power converted in the inverter is stepped down by the bidirectional converter and then the DC power is charged in the high-voltage battery. A variety of systems have been disclosed as power conversion devices applicable with the bidirectional converter.
A conventional chopper device as the power conversion device controls switching of a semiconductor switch connected between a DC power source and a load, to thereby output a DC voltage that is different to an input DC voltage. The chopper device includes: a voltage-error amplifier circuit for outputting a signal that is proportional to a deviation between a detection value of the output voltage of the chopper device and a preset value thereof; a conduction-time calculation circuit for calculating the conduction time of the semiconductor switch, proportionally to an input signal; a switching-command circuit, to which the output of the conduction-time calculation circuit is applied, for controlling switching of the semiconductor switch; and a conduction-time correction circuit provided between the error amplifier circuit and the conduction-time calculation circuit, for applying a correction signal to the conduction-time calculation circuit so that a ratio of the DC output voltage relative to the DC input voltage is proportional to the output signal of the voltage-error amplifier circuit (see, for example, Patent Document 1).
It is noted that, replacing the diode in the chopper device with a semiconductor switch makes possible bidirectional power conversion.
Further, a chopper circuit not including, on other than its input/output sides, any electricity storage element related to stepping up/down, but performing power conversion between a low-voltage side and a high-voltage side, like the above chopper device, will hereinafter be referred to as an SPC (Single Phase Chopper) circuit.
As a control method of the SPC circuit, there is disclosed a method of: performing PI (proportional-integral) control so that a difference between a target voltage and a voltage on the load device-side becomes smaller; and performing PWM (Pulse Width Modulation) control on two switching elements included in a bidirectional converter, irrespective of the stepping-up operation or the stepping-down operation (see, for example, Patent Document 2).
Meanwhile, as another conventional power conversion device, there is a device as described below.
The power conversion device has: multiple terminals; a reactor; a switching-element series circuit; a charging/discharging capacitor; and a smoothing capacitor. The multiple terminals has first, second, third and fourth terminals, and the switching-element series circuit comprises first, second, third and fourth switching elements connected in series to each other. The connection point between the second and third switching elements is connected through the reactor to the first terminal; the first switching element is connected, at its side opposite to its connection point with the second switching element, to the second terminal; the charging/discharging capacitor is connected between the connection point between the first and second switching elements and the connection point between third and fourth switching elements; the smoothing capacitor is connected in parallel to the switching-element series circuit; and the switching-element series circuit is connected to the third and fourth terminals. The first and second terminals are placed on a low-voltage side, and the third and fourth terminals are placed on a high-voltage side, so that DC voltage conversion is performed between the low-voltage side and the high-voltage side (see, for example, Patent Document 3).
It is noted that a chopper circuit including a charging/discharging capacitor on other than its input/output sides, and performing power conversion between a low-voltage side and a high-voltage side, like the above power conversion device, will hereinafter be referred to as an MLC (Multi Level Chopper) circuit.
On the other hand, for the purpose of high-power application, the capacitor used in the power conversion device is generally configured with plural capacitor elements that are connected in parallel to each other. It is known that, when the capacitor elements are connected in parallel to each other, parallel resonance by which the impedance becomes larger occurs among the series resonance frequencies of the respective capacitor elements. At the parallel resonance point, the impedance and the ESR (Equivalent Series Resistance) are likely to increase and thus, a circulation current flows inside the capacitor, so that abnormal heating is likely to occur (see, for example, Patent Document 4).